The use of outlet slots or openings at the muzzle end of the barrel of a firearm designed for fin-stabilized projectiles to allow firing gases to escape is known in the prior art. These openings may be placed either in the smooth-bore barrel end itself or in a separate portion that is attached to the end of the barrel. The openings generally have the form of outlet slots which extend either transversely or longitudinally to the barrel axis. When the firing gases escape through these outlet openings located at the muzzle of the barrel, the gas pressure behind a fin-stabilized projectile is considerably reduced. The end of the barrel provides guidance for the finned projectile and by lessening the gas pressure behind the fin-stabilized projectile, the gases are no longer able to exert a deleterious influence on the trajectory of the projectile as it leaves the barrel. In other words, there is an increased stabilizing affect on the projectile as it is fired from the barrel and the unfavorable jet effect caused by the high gas pressure in firearms of this type is eliminated through the use of such openings. The scatter and trajectory of the finned projectiles is improved providing increased accuracy.
Although the placement of openings at the muzzle end of the firearm has greatly increased the efficiency of such firearms, the deleterious affect of the escaping firing gases has not been completely eliminated. The highly compressed firing gases are still capable of escaping from the barrel before the fin-stabilized projectile has left the barrel end and at least a portion of the escaping gases has been found to continue to flow further along the outer circumference or periphery of the barrel end in the direction of the barrel axis. As a result, areas of turbulence form which adversely affect the fin-stabilized projectile immediately after it has left the muzzle opening in the barrel. These areas of turbulence are obviously non-uniform from one firing of the weapon to the next, and therefore the affect on the fin-stabilized projectile will vary from firing to firing so that the direction of the fin-stabilized projectile will be unfavorably influenced.
The trajectory of a projectile fired through a barrel having rifling on its interior surface is not affected by firing gases at the end of the barrel, the reason being that the rifling imparts a twisting motion to the projectile after it is fired and before it leaves the barrel. Thus, unlike a fin-stabilized projectile fired from a smooth-bore barrel, the rotation of the projectile ejected from a rifled barrel stabilizes the projectile and it is discharged from the barrel such that its trajectory will not be affected by any turbulence caused by the firing gases at the end of the barrel. A fin-stabilized projectile fired from a barrel which necessarily must have a smooth bore since it can't be rotated does not have the inherent stability of a rotating projectile.
The shape of a fin-stabilized projectile seen in longitudinal section differs substantially from the cartridge of a rifle. The rear portion of the fin-stabilized projectile is tapered, and thus the gases escaping from the barrel upon firing the projectile have considerable influence on the discharge of the fin-stabilized projectile whereby the fin-stabilized projectile may become inclined in one direction or the other thereby affecting its trajectory.